Introduction to CH-acidic Compounds

Keto-Enol Tautomerism

As clearly depicted by the resonance formulas, an enolate anion has two nucleophilic positions, namely the α carbon and the carbonyl oxygen. Enolate anions are ambident, that is, they possess two reactive (nucleophilic) centers. Even if the α carbon were to be more nucleophilic, as is particularly the case in reactions with soft Lewis-acid electrophiles, protonation may occur at the α carbon, as well as at the carbonyl oxygen, since a proton is an extremely hard Lewis acid. Consequently, protonation of an enolate may yield two different products, the enol (enolic form of a carbonyl compound), or the carbonyl compound (keto form).

The keto form and enolic form are in equilibrium. The establishment of equilibrium may be catalyzed by both acids and bases. Through suitable means, such as through fractional crystallization or careful distillation in the absence of any acid and any base, the keto and the enolic form may be separated from each other. The keto and enolic form of a carbonyl compound are constitutional isomers.

The separation must be carried out in the absence of all acids and bases, as the equilibrium reaction would otherwise proceed too rapidly. As a result, the separated, pure keto and enolic form would immediately be "contaminated" at least to some degree by the other form again. The glass of a flask is also capable of catalyzing keto-enol tautomerism. It must be noted that in contrast to an enol, the enolate is not in equlibrium with a keto form! Rather, the various structural formulas of an enolate, in which the negative charge is located at various positions, are actually merely resonance formulas of one and the same compound!

Fig.5

Acid-catalyzed keto-enol tautomerism.

Fig.

Experimental movie: Keto-enol tautomerism of ethyl acetoacetate.

Fig.6

Base-catalyzed keto-enol tautomerism.

Equilibrium position in keto-enol tautomerism

The position of the keto-enol equilibrium is influenced by the temperature and the solvent (if present). The keto form usually exceeds the enolic form to a considerable degree. Acetone, for instance, contains only 1.5*10-4% of the enolic form.

Fig.7

Phenol and its keto form.

Ketones are usually not enolized to such a degree as aldehydes are. However, β-dicarbonyl compounds are significantly much more enolized, as the double bond of a monoenolized β-dicarbonyl compound is additionally stabilized through resonance with the second carbonyl group. As a result, the α hydrogen between the two carbonyl groups of β-dicarbonyl compounds is much more acidic. The classic example of a compound that is virtually completely enolized is phenol. The equilibrium constant of keto-enol tautomerism equilibrium of phenol amounts to roughly 1010. It follows that the enolic form of phenol predominates to over 99.9 percent. Due to the strikingly high resonance stabilization of the aromatic system, the enolic form of phenol is much more stable than the non-aromatic keto form (cyclohexadienone).

Fig.8

Phenol as an acid.

In addition, phenol's enolate is much more stable than non-aromatic enolates are, as its negative charge is stabilized through resonance with the aromatic system. As a result, phenol is considerably more acidic than other enols or alcohols. The pKa value of phenol amounts to 9.95.